Reading a vernier caliper and a micrometer

The caliper and the micrometer are the two hand instruments every machinist reads, and both turn a coarse scale into a fine one by a trick: the caliper with a second, slightly shorter scale, the micrometer with a screw. NPL’s good practice guide and the Turkish Ministry of Education’s course material describe both the same way, and this page follows them. Hardness, another everyday shop-floor measurement, has its own page: hardness testing.

The vernier caliper

NPL describes it as having “a main scale, a vernier scale and a fixed and sliding jaw”; it measures outside, inside and depth. The fine reading comes from the vernier: “20 vernier divisions occupy 19 divisions on the main scale.” Each vernier division is then 0.95 mm, 0.05 mm shorter than a main-scale millimetre, so where the n-th vernier line meets a main-scale line the vernier zero sits n × 0.05 mm past the millimetre before it (MEB; NPL). Fifty divisions over 49 mm give 0.02 mm in the same way, ten over 9 mm give 0.1 mm (MEB), and NPL also lists a wide vernier of twenty divisions over 39 mm for 0.05 mm.

Reading takes three steps, the same in both sources: read the main scale just left of the vernier zero; find the vernier line that lines up exactly with a main-scale line; add the two. MEB’s examples: 10 mm and the ninth line of a 0.05 mm vernier make 10.45 mm; 12 mm and the 17th line of a 0.02 mm vernier make 12.34 mm. NPL reads “20 mm + 0.64 mm = 20.64 mm” on a 0.02 mm caliper. Before measuring, NPL closes the jaws and checks the zero.

Caliper 0.05 mmCaliper 0.02 mmMicrometer 0.01 mm

Measured value

—

mmTry another

1. Main scale, last line left of the vernier zero—
2. Vernier line that coincides with a main-scale line—
3. Reading—

Dial and digital calipers do the same job with less reading. NPL gives typical resolutions of 0.05 mm for vernier, 0.02 mm for dial and 0.01 mm for digital calipers, and Mitutoyo writes that “Smaller vernier calipers have largely been supplanted by dial or electronic calipers”.

The outside micrometer

A micrometer reads a screw. Its spindle has a 0.5 mm pitch and the thimble 50 divisions (MEB; NPL): “Each thimble graduation therefore equals one hundredth of a millimetre.” (NPL). The sleeve carries 1 mm lines above the index line and 0.5 mm lines below it (NPL). To read it, “the value of the nearest visible graduation on the sleeve to the thimble is determined” and the thimble division on the index line is added (NPL). MEB’s example: 5.50 mm on the sleeve and 35 thimble divisions make 5.85 mm; in an Ankara University lab sheet the thimble edge sits between 23.5 and 24.0 mm and division 21 is on the line, so 23.71 mm. NPL estimates a third decimal from the line itself, since “the width of a thimble graduation line equals one fifth of a thimble division”; some micrometers carry a vernier on the sleeve instead (NPL; Mitutoyo).

To try it yourself, use the micrometer reading simulator: it has a step-by-step tutorial and a test on random parts.

The micrometer has what the caliper lacks: a device for a constant measuring force. NPL: the ratchet slips and clicks at its limit, and “Always take the reading on the first click of the ratchet”. Mitutoyo compares the ratchet stop, with “tactile and audible feedback” but usually needing two hands, and the friction thimble, with “a somewhat ‘mushy’ feel and a lack of audible feedback”. The zero comes first: “Confirm that the zero line on the thimble is aligned with the index line on the sleeve” (NPL). Above 25 mm, where the spindle cannot reach the anvil, Mitutoyo uses setting standards to “verify (not calibrate) the zero opening position”.

Resolution is not accuracy

The smallest step an instrument shows says little about how right it is. NPL works through a 0.02 mm caliper with its face flatness, parallelism and squareness, five readings between 20.06 and 20.12 mm, and arrives at an expanded uncertainty of 0.035 mm, nearly twice the resolution. Makers’ datasheets show the same gap: Mahr gives its MarCal 16 ER digital caliper a resolution of 0.01 mm and an “Error limit (mm) 0.03”; Mitutoyo gives one 0–25 mm Digimatic micrometer a resolution of 0.001 mm and an accuracy of ±2 µm, ”* Excluding quantizing error of ±1 count”. Both are maker figures for single models; Mahr’s Turkish datasheet gives a 175–200 mm Micromar 40 A micrometer an error limit of 7 µm, likewise a maker figure.

The standards for the two instruments are ISO 13385-1:2019 for calipers, which covers analogue (“vernier scale or circular scale (dial)”) and digital indication (SIS), and ISO 3611:2023 for micrometers, whose third edition added a “classification system of maximum permissible errors” and a reference to ISO 14978:2018. NPL noted in 2014 that, under ISO 14978, equipment standards of this kind “shall not include any numerical values for MPEs and MPLs”, so the numbers are in makers’ datasheets; whether the 2023 classification contains numbers was not visible in the parts of the standard read for this page. For choosing an instrument, Mahr’s Gaging Tips column gives a rule of thumb: “the measuring instrument should resolve to approximately 1/10 of the tolerance being measured”.

Errors in use

• Abbe’s principle. NPL: “maximum accuracy may be obtained only when the standard is in line with the axis of the object being measured”, and “The construction of the calliper does not conform to Abbe’s principle”; KEYENCE adds that “outside micrometers follow Abbe’s principle”. NPL’s advice for the caliper: measure as close to the beam as possible.
• Force. “Unlike micrometers, most vernier callipers are not provided with a device that allows a constant measuring force.” Pressed too hard, the slider’s leaf spring “will bend, causing the jaw slider to tilt” (NPL).
• Parallax. On a vernier “the apparent alignment position will vary slightly depending on the viewing angle”; on a micrometer, “view the index line on the sleeve from directly above” (NPL).
• Temperature. “Lengths of objects are determined at the internationally adopted temperature of 20 °C”, and “A 100 mm (4 in.) long iron rod will change length by 0.012 mm (0.000 47 in.) with a temperature change of 10 °C” (NPL). For the micrometer, NPL asks for instrument and part to stand in the room for at least an hour to reach room temperature; for the caliper, it asks for the temperature difference between caliper and part to be kept as small as possible. It warns against measuring parts still hot from machining.